Position sensors are a common element in automotive, industrial, and aerospace applications. Whenever safety is a concern, it is an absolute must that highly robust and reliable position sensors are required. Potentiometers are used as position sensors. They are contact-type sensors and lead to wear and noise. To overcome these drawbacks, non-contact type sensors are used. These sensors are based on inductive, capacitive, optical, and Hall Effect principles. Optical encoders provide good resolution but lead to higher cost and reliability related issues in the harsh/contaminated environment. Hall Effect sensors are sensitive to temperature and external magnetic fields. Capacitive sensors are very sensitive to extreme environmental changes.
Inductive sensors are used to convert a linear displacement or an angular motion of a conductive target into a proportional electrical signal using currents generated by a magnetic field induced in one or more sensing coils. Some inductive position sensors include at least one primary coil that sustains an oscillating signal producing a magnetic field and one or more secondary coils that receive the position information as currents induced by the magnetic field as a function of position of the conductive target.
Conventional inductive position sensors are expensive, and they occupy more space as the oscillator and sensor coils are radially wound on a core. Planar inductive position sensors are cost-effective as coils are laid out in a plane on a printed circuit board.
Planar inductive sensors consist of one or more oscillator coils, two sensing coils and one movable conductive target which influences the strength of magnetic coupling between the one or more oscillator coils and the sensing coils. An alternating current is induced through the one or more oscillator coils. The magnitude and phase of this alternating current depend on the position of the target. The eddy currents through the conductive target cause a difference in sense coil currents and voltages. Attempts are made to configure the sensing coils such that in a uniform magnetic field not influenced by a conductive target the voltages and currents induced in the sensing coils cancel each other out.
A few examples of prior-art planar inductive position sensors include United States Patent Publication US 20050253576, U.S. Pat. Nos. 4,507,638, 6,522,128, 7,196,604, and WO2002097374. A planar inductive sensor having two sensing coils and a pair of oscillator coils wrapped around the sensing coils is disclosed in Application Note AN-S1412 “Inductive Sensor Coil Design Using LX3301A” (2017) from Microsemi Corporation of Chandler Ariz.
One drawback of available linear inductive planar position sensors is that there is always a non-uniform magnetic field that exists at the edges of the sensor. This non-uniform magnetic field at the edges of the sensor causes an offset voltage to be induced in the sensing coils, which negatively effects the accuracy of sensor.
Generally, edge effects in linear sensors can be minimized by placing the sensing coils far away from oscillator coil edges, but this technique is not feasible in space-constrained applications because it increases the sensor printed circuit board size.
Referring first of all to FIG. 1, a diagram shows an example of a prior-art planar inductive linear position sensor 8 formed on a substrate 10, including an oscillator coil having a pair of oscillator coil segments 12 and 14 driven from a center tap, a sine sensing coil formed from two 360° segments 16a and 16b, the segment 16a starting from the left side of FIG. 1 at 0° (sin 0°=0) and the segment 16b starting from the left side of FIG. 1 at 0° (−sin 0°=0) and joined at their end points 42, 44, a cosine sensing coil formed from two segments of opposing phase shown in dashed lines at reference numerals 18a and 18b, the segment 18a starting from the left side of FIG. 1 at 0° (cos 0°=1) and the segment 18b starting from the left side of FIG. 1 at 0° (−cos 0°=−1) and joined at their ends by segments 46, 48, a movable conductive target 20. The movable target 20 is formed from a conductive material and is preferably formed from a material having a relatively high electrical conductivity, such as copper or aluminum. The sine sensing coil 16a and 16b is shown having leads 22 and 24 and the cosine sensing coil 18a and 18b is shown having leads 26 and 28.
Persons of ordinary skill in the art will appreciate that while the particular prior-art configuration shown in FIG. 1 employs a pair of oscillator coils 12 and 14 that may be conveniently formed from a single coil center tapped at reference numeral 30 from which it is driven by a signal Vin and having end leads 32 and 34, other prior art configurations employ a single oscillator coil driven by a suitable signal generator.
As is known in the art, the pair of oscillator coils 12 and 14, the sine sensing coil segments 16a and 16b, and the cosine sensing coil segments 18a and 18b can be formed as separate layers on multilayer substrate 10 using conventional printed circuit board fabrication techniques.
The oscillator signals may be generated by and the sensed signals may be received and processed by sensor interface circuitry, for example, a single sensor interface integrated circuit 36 such as a LX3301A Inductive Sensor Interface integrated circuit, available from Microsemi Corporation of Chandler, Ariz. Such a sensor interface circuit can include a signal generator section 36a used to generate the oscillator signal that is injected into the one or more oscillator coils, and sensing circuits 36b and 36c for sensing signals from the sine and cosine sensing coils, respectively. Capacitors 38 and 40 are coupled, respectively, between the end leads 32 and 34 of the oscillator coils 12 and 14 and ground to form LC resonant circuits. A typical value for capacitors 38 and 40 can be about 1.2 nF. The oscillator signal shown as Vin injected into the oscillator coil segments 12 and 14, ends of which are shown in FIG. 1 connected to the signal generator section 36a of the inductive sensor interface integrated circuit 36 at connections Osc. 1 and Osc. 2, respectively, is preferably a sine wave and the frequency of the oscillator signals injected into the oscillator coils depends only on the inductance of the oscillator coils 12, 14 and the respective capacitance values of capacitors 38 and 40. A typical, non-limiting oscillator frequency range can be between about 1 MHz and about 6 MHz.
The voltage induced by the movable conductive target 20 in one of the sine or cosine sensing coils is a time-dependent derivative of the magnetic flow from Maxwell's equation
  E  =      -                  d        ⁢                                  ⁢        φ        ⁢                                  ⁢        B                    d        ⁢        t            
Faraday's law of induction makes use of the magnetic flux ΦB through a region of space enclosed by a wire loop. The magnetic flux is defined by a surface integralØB=∫B·dA 
An alternating current Io(t) is applied to the oscillator coil segments 12, 14 that creates an alternating magnetic field Bt(t). The alternating magnetic field Bo(t) induces in the movable conductive target 20, which in a simplified form is a closed conductive loop, a current It(t) that, in turn, creates an alternating magnetic field Bt(t) that opposes the exciting alternating magnetic field Bt(t).
A voltage is induced in each of the sine and cosine sensing coils from the overlapping alternating magnetic fields Bo(t)+Bt(t) according to the relationship
  E  =            -                        d          ⁢          ∅                dt              =                  -                              d            ⁢                          ∫                              ∫                                  (                                      (                                                                                            B                          0                                                ⁡                                                  (                                                      t                            ,                            x                            ,                            y                                                    )                                                                    -                                                                        B                                                      t                            ⁡                                                          (                                                              t                                ,                                x                                ,                                y                                                            )                                                                                                      ⁢                        d                        ⁢                        A                                                                                                                          dt                    =                        -                      d            dt                          ⁢                  ∫                      ∫                          (                                                                                          B                      0                                        ⁡                                          (                                              t                        ,                        x                        ,                        y                                            )                                                        ⁢                  dA                                +                                                      d                    dt                                    ⁢                                      ∫                                          ∫                                                                        (                                                                                    B                              t                                                        ⁡                                                          (                                                              t                                ,                                x                                ,                                y                                                            )                                                                                )                                                ⁢                        d                        ⁢                        A                                                                                                                                    
with A representing a surface area of the respective sensing coil.
It should be noted that each of the sensing coils have two surfaces, a positive lobe and a negative lobe, directed in opposite directions. The voltage induced in the positive lobe is Ep and the voltage induced in the negative lobe is En. The result is that, with a sine sensing coil having a symmetrical coil geometry as seen easily in FIG. 2, the portion of the induced voltage that is caused by the oscillator coil segments 12, 14 is zero; that is
                                          d            dt                    ⁢                      ∫                          ∫                                                B                  0                                ⁡                                  (                                      t                    ,                    x                    ,                    y                                    )                                                                    )            ⁢      dA        =    0              i      .      e      .        ,                            E          p                -                  E          n                    =      0      
When a current carrying conductor is placed in parallel with another conductor, there will be a magnetic coupling between the two conductors, resulting in additional induced voltage. Because the ends of the sine sensing coil segments 16a and 16b closest to the end portions of the oscillator coil segments 12, 14 are joined together at points 42 and 44 they present very little, if any, conductors having significant lengths that are close enough to form parallel conductors that will magnetically couple with the conductors forming the end portions of the oscillator coils 12 and 14.
In the case of the cosine sensing coil segments 18a and 18b there is always an edge effect resulting from its end segments 46 and 48 which are provided to connect the cosine segments 18a and 18b together at their ends, as can be seen in FIG. 3. These end segments 46 and 48 are oriented in parallel with the conductors that form end portions of the oscillator coils 12 and 14, and will be magnetically coupled to the conductors that form end portions of the oscillator coils 12 and 14. This coupling results in additional induced voltage (Ep+ΔEp) being coupled into the positive lobes of the cosine coils 18a and 18b from the oscillator coils 12 and 14 compared to voltage (En) induced in the negative lobe of the cosine sensing coil 18a and 18b. Because of this always there is an offset voltage ΔEp induced in cosine sensing coil, which means
                    d        dt            ⁢              ∫                  ∫                                                    B                0                            ⁡                              (                                  t                  ,                  x                  ,                  y                                )                                      ⁢            dA                                ≠    0              i      .      e      .        ,                            (                                    E              p                        +                          Δ              ⁢                              E                p                                              )                -                  E          n                    ≠      0      
The asymmetrically produced voltage on the cosine sensing coil adds the offset voltage to the voltage coupled in from the movable target. This creates difficulties for further processing the signal and introduces substantial measuring errors.
FIG. 4 is an amplitude vs. position plot of demodulated waveforms from the planar inductive linear position sensor 8 of FIG. 1, showing the waveform 50 sensed from the sine sensing coil 16a and 16b and the waveform 52 sensed from the cosine sensing coil 18a and 18b of the sensor of FIG. 1. As can be seen from FIG. 4, there is an induced offset voltage indicated at reference numeral 54 in the demodulated cosine waveform 52 of FIG. 4.